In recent years, light weight plastic automobile energy absorbing units (EAUs) have been employed in conjunction with conventional metal automobile bumpers to absorb low energy (lower speed) impacts. The plastic automobile EAUs are usually employed in front of the bumpers. Plastic automobile EAUs are usually composed of a core material of a plastic foam and a surface material of a synthetic resin enclosing the foam core and are usually mounted on a rigid cross beam. Plastic materials which have been proposed for the foam core material include polyurethane, polyethylene bead, polystyrene and polypropylene bead foams. The foam core material is an important component part which affects the performance of an automobile EAU, and is generally required to have excellent energy absorbing properties and dimensional recovery. Further, to decrease the weight of an automobile, the EAU core material should be of low density.
Polyurethane foam EAU cores have excellent energy absorbing properties and dimensional recovery, but have relatively high density. As a result, the polyurethane foam EAU cores are relatively heavy and expensive. Polystyrene foam has relatively poor dimensional recovery and impact strength. Accordingly, automobile EAU cores made of foamed beads of an olefinic polymer, especially a polypropylene-type resin, are most common.
A conventional automobile EAU core of foamed olefinic polymer is prepared by feeding expandable beads of a polypropylene-type resin in a mold capable of enclosing the particles but allowing escape of gases therefrom, and heating the expandable beads at a temperature at which the beads expand and soften and melt-adhere to each other into a mass, whereby a foamed molded article expanded to the same shape and size as the cavity of the mold is formed. An automobile EAU core prepared from beads of a polypropylene-type resin foam requires a foam density of about 12.5 pounds per cubic foot to achieve a compressive strength of about 250 pounds per square inch at 25% strain. Automobile EAU cores having comparable compressive strength at a lower density would be desirable.